1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method therefor, and more particularly to a bipolar transistor type semiconductor device and a manufacturing method therefor.
2. Description of the Related Art
In comparison with a field-effect transistor, a bipolar transistor has a large current-driving power and a high cut-off frequency, so that it is suitable for use in a high-speed electric circuit. In order to drive a bipolar transistor at high speed, parasitic resistance and parasitic capacity must be reduced as small as possible. Effective measures taken for this, purpose are: (1) to reduce the base resistance by narrowing the emitter; and (2) to reduce the base-collector capacity by narrowing the base-collector region. In consideration of these measures, various self-alignment techniques have been developed with respect to Si-based, double-diffusion type transistors, particularly to double-diffusion type transistors wherein the emitter and base electrodes are formed of polycrystalline silicon. As a result of the self-alignment techniques, the constructions of transistors have been improved, and accordingly, semiconductor devices have come to achieve higher operation than before. At the present time, however, the techniques for achieving higher operation of semiconductors seem to have reached their limits.
In recent years, attention of those engaged in the art is attracted to the use of an emitter having a wide forbidden band gap, more specifically to the use of an emitter formed of a semiconductor material whose forbidden band gap is wider than that of the material of the base. If the emitter is formed of such a semiconductor material, the emitter injection efficiency is not adversely affected even if the base doping concentration is increased, so that the base resistance can be reduced remarkably. This type of transistor, which is generally referred to as a heterojunction bipolar transistor (hereinafter abbreviated as HBT), is usually developed in the MBE or MOCVD process, using mainly semiconductor compounds obtained from the elements of groups III-V of the Periodic Table since such elements produces satisfactory heterojunctions and ensures good mobility of electrons. Like the case of an Si-based bipolar transistor, the widths of the emitter and the base-collector region of the HBT must be decreased, so as to achieve high operation. With the lithography technique presently available, however, the minimum stroke width and the allowance of mask alignment are substantially the same. Therefore, the self-alignment technique which needs no allowance of mask alignment is indispensable for achieving minute HBTs.
Among the presently known self-alignment techniques for HBTs, the technique used in the HBT manufacturing method using AlGaAs-GaAs material and proposed by Dumke et al. ("Solid State Electronics" Vo. 15 (1972), pp 1339-1343) may be convenient since it needs no allowance of mask alignment when emitter and base electrodes are formed.
This technique is still inappropriate to practical use, for the two reasons below.
(1) In the technique, a eutectic reaction between Au (of which to form an emitter electrode) and Ga occurs even at normal temperature. Therefore, when the AlGaAs layer is etched by use of an AuGe alloy mask, so as to form an emitter layer, abnormal etching may take place at the interface between AuGe alloy and the AlGaAs layer As a result, it is difficult to obtain an emitter layer having a desirable sectional shape.
(2) In order to produce a uniform undercut below the emitter electrode, an emitter stripe has to be arranged in the (1,1,0) direction. This arrangement is not desirable, in view of high integration of a transistor.
As mentioned above, various proposals have been made to obtain a semiconductor device which enables high-speed operation and utilizes the characteristics of a bipolar transistor (e.g., a high cut-off frequency characteristic). However, since the proposals are not satisfactory in their manufacturing processes, the semiconductor devices available at present do not fully satisfy the requirements in practical use.